Cheaper alternatives to MS Exchange Server?

I’m looking into a cheap alternative to MS Exchange Server for my company.

We’ve already got a mailserver, but I’m looking for something that can utilize a shared company calendar, and a common address book.

That’s really all we need it for - everything else in the company works just fine, and we’re loathe to commit so much money to bloatware like Lotus Notes that has way too much functionality.

Any ideas?